In the case of satellite television, the converters are generally called “LNBs”, the abbreviation standing for “Low Noise Block”; when the system comprises two converters, they are generally each placed at the center of a satellite antenna and thus there are two antennas: however, they may sometimes be placed at the center of the same satellite antenna, according to a so-called “twin LNB” configuration.
The document EP 0314931 (BRIONVEGA) describes a system comprising a plurality of converters and antennas, a single tuner (“satellite receiver” referenced 21), and means (referenced 29) for switching the input of this tuner by choice over to one or other of the plurality of converters (referenced 4, 7).
In the case of a system utilizing two converters and two tuners, the simplest configuration consists in connecting a converter to the first tuner and the other to the second tuner; according to a more complex configuration, the first converter provides a signal to be split on the one hand permanently to the first tuner and on the other hand optionally to the second tuner, according to the position of these switching means; such a system then generally comprises:                switching means exhibiting two possible positions, one for switching the input of the second tuner over to the second converter as in the simplest configuration, the other for switching the input of the second tuner over to the first converter,        splitting means (the so-called “splitter”) for splitting the signal delivered by the first converter to two outputs, one of which is permanently linked to the input of the first tuner, and the other of which is linked to the switching means; these splitting means are generally incorporated into the first tuner, so that this tuner then exhibits, in addition to the input for gathering the signal from the first converter, an output for duplicating this signal to another tuner.        
A two-tuner system as described above may have the following applications:                it allows the viewer to watch a television program picked up by the first converter and the first tuner, while recording another television program processed by the second tuner; such an application is described in particular in patent application EP300193—THOMSON.        it allows the viewer to watch a main image of a television program arid to inlay a secondary image of another television program into this image; such an application is described in particular in patent application U.S. Pat. No. 5,557,338—THOMSON.        
By virtue of the switching means of the system described hereinabove, the system can therefore be used, as described hereinabove, to receive two television programs simultaneously, whether these programs originate from two different transmitting sources via the two converters of the system, or whether these programs originate from the same transmitting source via just the first converter of the system:                in the first case of different sources, the switching means are actuated in such a way that the input of the second tuner is linked to the second converter,        in the second case where the source is identical, the switching means are actuated in such a way that the input of the second tuner is linked to the first converter via splitting means as described hereinabove.        
For applications of this type, it is then necessary to actuate the system switching means, depending on whether the two programs originate from the same source or from different sources.
To actuate the switching means, one merely needs to change the arrangements for plugging in the input of the second tuner.
To avoid this manual operation, it is also possible to provide switching means which can be actuated remotely, for example with the aid of a remote control; such switching means are described (and referenced 29) in the document EP 0314931 already cited and are actuated remotely by the keypad (referenced 23) of a remote control (see column 5, lines 3 to 6).
The intervention of an operator for the actuation of the switching means has drawbacks, related for example to the risk of a keying error or to the need, for the operator, to know whether the two programs are or are not broadcast by the same source.
The aim of the invention is to avoid these drawbacks.
Document U.S. Pat. No. 6,009,304 describes a system comprising two RF low signal noise converters suitable for delivering by choice a left polarized signal (“L”) and/or a right polarized signal (“R”); each converter comprises a low noise amplifier (“LNA”), a bandpass filter (“BPF”), a mixer (“MIX”) for converting the incoming signal into an intermediate frequency signal, and an intermediate frequency amplifier (“IF Amp”); between the mixer (“MIX”) and the amplifier (“IF Amp”), the system comprises means (“S.W.”) for switching the input of each amplifier (“IF Amp”) by choice over to the output of the mixer of the first converter or/and over to the output of the mixer of the second converter; the system comprises means for driving the switching means as a function of the value of the voltages applied to the output of the converter (see claim 3+column 1, lines 40 to 60); unlike in the case of the reception systems described above, the state of the switching means is, at this juncture, independent of the state, in service or otherwise, of the converters; the system finally comprises means suitable for putting one or other of the converters out of service by removing the power supply to its intermediate frequency amplifier (“IF Amp”), when it is detected that the output of this amplifier is not loaded (claim 1); the output of an amplifier is considered not to be loaded when a voltage of 0 V or less than the voltage for driving the switching means is measured (column 5, lines 29 to 35).
According to this document, the same means of detection serve to drive switching means and means for placing out of service; the detection means are not suitable for detecting whether one or other of the converters is in service; thus, this document does not teach any solution of a kind as to avoid the aforesaid drawbacks.